NOVEL COBALT CHROME ALLOYS AND PREPARATION

Abstract: New alloys of cobalt chromium having more ceramic-like properties than have been previously obtained in other alloys of CoCr are provided. Their methods of preparation and articles of manufacture especially for orthopedic applications (for example in knees and hips) are given.

Description: NOVEL COBALT CHROME ALLOYS AND PREPARATION Background of the Invention Field of the Invention This invention relates to new alloys of cobalt chromium (CoCr) (also often referred to herein as "cobalt chrome") and their preparation, and it relates in particular to new alloys of cobalt chrome having improved properties and being suitable for use against ultra high molecular weight polyethylene (i.e., UHMWPE) surfaces in opposing friction couples (for example in knees and hips in the field of orthopedics). In the prior art, pure ceramic zirconia femoral heads for use in artificial hips are commercially available. These devices have some disadvantages, however. For example, the materials are expensive and they are more susceptible to fracture than metal heads. Also known in the prior art are zirconium alloy femoral heads. These are made, for example, by Smith-Nephew Company. These also have the disadvantage that they are very expensive. It is also known to implant nitrogen ions into cobalt chrome and into titanium alloys for applications in the field of orthopedics. However, nitrogen implantation does not produce good wear properties for titanium alloys against UHMWPE. See for example H.A. McKellup, T.NΛ Rostlund, J. of Biomedical Materials Research, vol. 24, pp. 1413-1425 (1990). Therefore, new materials with improved properties were sought, especially, for orthopedic applications. An object of this invention is a new material with improved properties to be used against ultra high molecular weight polyethylene (UHMWPE) surfaces in opposing friction couples (for example in knees and hips). Another object of this invention is a method of making a cobalt chrome alloy that exhibits more ceramic-like properties than have been previously obtainable in such alloys. Yet another object of this invention is a new and improved cobalt chrome alloy and articles of manufacture (e.g. knee replacements and femoral heads) prepared therefrom. Summarv of the Invention These and other objects are satisfied by the method of the invention of producing a new cobalt chrome alloy comprising: (a) implanting a substrate of cobalt chrome alloy with zirconium ions (preferably from a pure zirconium target) so as to produce a new cobalt chrome alloy implanted with zirconium. Additionally and optionally, the new zirconium implanted cobalt chrome alloy can be coated (if desired) with a coating of zirconium. When the cobalt chrome alloy is in the form of an orthopedic device, the method further comprises subjecting the device to sterilization procedures for example by gamma irradiation, which are known in the art. Also according to the invention, a new cobalt chrome alloy is provided comprising cobalt chrome which has been treated by implanting the surface of the cobalt chrome with zirconium. This alloyed surface is then permitted to oxidize through contact with ambient air. It is believed that zirconium ions have not previously been implanted into any material and in particular not into cobalt chrome alloy. It has been found that a very thin gradient of zirconium metal adds life to the part which is subjected to wear, in spite of the shallow depth of implantation with zirconium. Additionally, the cost of manufacture is minimized by reducing the quantity of zirconium needed to effect the observed improvements. Unexpectedly, it has been found that a very desirable combination of properties are obtained for the cobalt chrome alloy which has been implanted with zirconium ions as compared with the cobalt chrome alloy not subjected to implantation by zirconium. These properties include improved resistance to abrasion of the cobalt chrome alloy by bone cement; improved resistance of the alloy to corrosion by saline; and reduced wear of the alloy when articulated against the surface of UHMWPE. Description of the Preferred Embodiments Brief Description of the Drawings Figures 1-3 are plots of applied potential versus current for zirconium implanted CoCr alloy (see Example 1 described below), untreated CoCr (Vitallium)(see Example 1 described below), and zirconium coated CoCr alloy (see Example 2 described below), respectively. (See Example 1 , described below). Fig. 4 is a plot of total weight loss of UHMWPE vs. number of complete cycles in a reciprocating wear test for zirconium implanted CoCr (described below in Example 1). In the practice of the invention, when the new material is to be used for subsequent orthopedic purposes, any cobalt chrome alloy can be used as the substrate to be implanted with zirconium, provided that it is manufactured under the procedures of ASTM standard F75, which describes the standard procedures for producing orthopedic cobalt chrome alloys. One such suitable cobalt chrome alloy is produced by Howmedica Inc. and is sold under the trade name of Vitallium ® . The ASTM standard procedure F75 for the preparation of cobalt chrome alloy is hereby incorporated herein by reference. The zirconium ions are implanted into the cobalt chrome alloy by the following procedure. Pure zirconium is used as the target. The general procedure for performing ion implantation into a substrate is described in the article by James R. Treglio, et al., "Performance of the Advanced MEWA ® IV 80-10 Metal Ion Implantation System", Surface and Coatings Technology, 51., (1992), 546-550. That article is hereby incorporated herein by reference. The general procedure described in that article was used. However, any of a number of standard commercially available ion implantation technologies will result in zirconium implantation of the CoCr surface. In the practice of the invention, when the cobalt chrome substrate is an orthopedic device (for example a hip or a knee or other article of manufacture), the article of manufacture (following the step of zirconium ion implantation) must be subjected to sterilization procedures (for example, by gamma irradiation). Examples The following Examples demonstrate the zirconium modification of cobalt chrome alloys and associated experimental data. Example 1 Zirconium Implantation of Cobalt Chromium (ASTM 799) Alloy Ion implantation, using Zirconium as the implant species, was carried out using an MEWA IV (Metal Vapor Vacuum Arc) ion source as a commercial source, by ISM Technologies, San Diego, California. The MEWA ion source generates a plasma via a cathodic arc. Highly polished (Average roughness, R a < 100 A) Cobalt Chromium discs, one inch in diameter, and curved wear test specimens were placed in the chamber 90 cm from the ion source, aligned such that the center of the discs was at the approximate center of the beam. The vacuum chamber was pumped down to a base pressure of 3x10 '7 Torr by an oil free roughing pump and a cryogenic pump. The combination results in an oil free system, precluding any hydrocarbon contamination of treated surfaces. Zirconium was implanted into the discs using an accelerating voltage of 70 kV to a minimum dose of 2x10 17 /cm 2 . This produced a surface implanted with Zirconium metal atoms to a depth of approximately 800 A. The implanted Cobalt Chromium discs were evaluated to determine their coefficient of friction in bovine serum, corrosion resistance in saline, universal vickers hardness, and abrasion resistance to bone cement. Coefficient of Friction The coefficient of friction for the UHMWPE - Zirconium implanted CoCr wear couple was determined using a commercially available pin-on-disc tribometer manufactured by Implant Sciences of Wakefield, MA. Conditions and the results of this testing are listed in Table 1 A. Table 1A - Coefficient of Friction Test Conditions: Equipment: ISC-200PC Pin-on-Disc Tribometer Pin: UHMWPE, radius 6.35 mm Disc: 800 A implanted CoCr Disc Lubricant: Bovine Serum Atmosphere: Ambient Temperature: Room Temperature Load: 50 grams Speed: 1.74 cm/sec Sample Coefficient of Friction Zirconium implanted CoCr 0.06 Bulk Zirconia 0.04 CoCr Control 0.02 The implantation of CoCr with Zirconium has resulted in a coefficient of friction more like that of bulk Zirconia than that of cobalt chrome. Corrosion Resistance The resistance of the Zr implanted CoCr to accelerated corrosion in saline was evaluated using the anodic polarization technique. A fixed area (1 cm 2 ) of the disc was placed in saline, and current flow was monitored while increasing applied potential. Table 1 B lists the test conditions and results. Table 1B - Corrosion Testing Test Conditions: Experimental technique: Anodic Polarization Test Solution: 0.9% deionized, deaerated saline Scan Rate: 0.17 mv/sec Sample Corrosion Potential Breakdown Potential (versus SCE) Zirconium implanted CoCr -0.104 V 0.85 V CoCr Control -0.067V 0.65 V The Zirconium implanted CoCr showed a corrosion potential lower than the CoCr control. Additionally, the Zirconium implanted CoCr had a higher resistance to breakdown of the surfaces protective film than the CoCr control. When the potential was reversed, the Zirconium implanted CoCr sample showed immediate repair of the protective film, similar to the control, as indicated by an immediate decrease in current flow. (See Figures 1 and 2). The results of this test indicate the Zirconium implanted CoCr has improved corrosion resistance over the untreated CoCr. Hardness Using a Fischerscope H100V microhardness tester, the universal hardness values were determined for both the Zr treated and untreated CoCr. The Zirconium implanted sample had an ISE corrected hardness of 5936 ± 588 N/mm 2 . Untreated wrought CoCr had a hardness of 5481 ± 188 N/mm 2 . Thus the implantation did not result in significant hardening of the surface; however, this technique may be insensitive to such a shallow implantation depth. Thus the performance of the implanted material during wear and abrasion testing cannot be a result of increased surface hardness. Abrasion Testing The Zirconium implanted CoCr discs were evaluated for their resistance to abrasion by Simplex bone cement. The flat discs were abraded using a hemispherical bone cement pin reciprocating against the disc under an applied load of approximately 1.25 Kg using water as a lubricant. The discs were checked for any resulting weight loss and any increased surface roughness. The pins were similarly checked for weight loss. The average weight loss for two treated discs was 0.22 mg, compared to 0.560 mg for an untreated control. The surface scar resulting from the abrading pin against the polished flat was much less for the Zr treated sample than for the untreated control. The Zr treated sample had a scar an average of 300 A deep, compared to a scar of 1 ,980 A for the untreated sample. Pins abrading against the Zr treated samples showed a weight loss of 4.9 mg, compared to 6.2 mg for the untreated controls. Thus the Zr implanted CoCr discs demonstrated significantly increased resistance to abrasion by bone cement, compared to untreated samples. This was not expected, based upon no significantly demonstrated improvements in hardness or coefficient of friction. Wear Testing Forged Cobalt Chromium curved wear testing specimens were ion implanted to a depth of 800 A with Zirconium as a commercial service by implant Sciences, Wakefield, MA. The Zr implanted CoCr was loaded and reciprocated in 30% bovine serum against UHMWPE. The experimental conditions are listed in Table 1C. Table 1 C - Wear Testing WEAR TEST CONDITIONS EQUIPMENT: Reciprocating wear machine LOAD: 250 lbs FREQUENCY: 1 Hertz OSCILLATING AMPLITUDE: ± 60 deg DISTANCE PER CYCLE: 2.96 inches LUBRICANT: 30% Bovine Serum diluted with Deionized water + 0.3% Sodium Azide TEST COUNTER SURFACE: UHMWPE irradiated at 2.5 Mrads, presoaked for 4 weeks After 2.4 million cycles the weight losses of the UHMWPE counter-surfaces were measured. The UHMWPE test specimens articulating against the zirconium implanted sample lost only 0.953 mg, compared to an average of 1.12 mg for unimplanted CoCr controls. (See Figure 4) The untreated CoCr samples showed random scratches parallel to the wear direction, whereas the zirconia implanted CoCr specimens showed only minor scratches. Thus the zirconia implanted specimens reduced the wear of the UHMWPE counterfacing surface and exhibited improved resistance to abrasion. Example 2 Highly polished CoCr discs were ion implanted as per Example 1 , with an additional deposition of a 2500 A coating of Zirconium. These coatings were tested for corrosion and abrasion resistance, coefficient of friction, and universal hardness. Corrosion Resistance The samples were tested using the identical conditions that were used in Example 1. Table 2 - Corrosion Testing Anodic Polarization of CoCr coated with 2500 A Zirconium Sample Corrosion Breakdown Potential (Volts) Potential (Volts) Zr Coated CoCr -0.390 0.95 CoCr Control -0.067 0.65 The Zirconium coated samples showed much lower corrosion potential and much higher breakdown potential than untreated CoCr (See Figure 3). This would indicate a significant improvement in resistance to corrosion in a saline environment. Hardness Using a Fischerscope H100V microhardness tester, universal hardness values of 5303 ± 278 N/mm 2 were determined for the Zirconium coated samples. Untreated forged CoCr had a hardness of 5481 ± 188 N/mm 2 . Bulk Zirconia, under identical test conditions had a hardness of 11709 ± 590 N/mm 2 . Thus, the Zirconium coated samples probably reflect the hardness of the CoCr substrate because of the thinness of the surface coating. Coefficient of Friction Under the same conditions as listed in Example 1 , the Zirconium coated samples had a coefficient of friction of 0.12. Abrasion Resistance Zirconium coated CoCr discs were tested for resistance to abrasion by bone cement under conditions identical to those listed in Example 1. After 1 million cycles, the Zirconium discs showed almost no visible scratching and had a weight loss of 0.06 mg (average), compared to 0.36 mg for untreated controls. This was an unexpected result since the coating shows a composite hardness (coating + substrate) less than the untreated CoCr and a coefficient of friction higher than the untreated CoCr control.

Claims: CLAIMS 1. A composition comprising a cobalt chrome alloy having implanted therewith zirconium metal (not a coating). 2. A composition according to claim 1 and including also a coating of zirconium. 3. A cobalt chrome alloy prepared by implanting a substrate of cobalt chrome alloy with zirconium ions from a pure zirconium target. 4. A composition according to claim 3, wherein said pure zirconium metal has been ionized, the ionized zirconium extracted and accelerated in order to impact the desired target with sufficient energy to implant zirconium into the surface of said cobalt chrome alloy. 5. A composition according to claim 4 wherein said surface has oxidized over a period of time. 6. A composition according to claim 5, and including also a coating thereon of zirconium. 7. A composition according to claim 3, wherein said pure zirconium is implanted in an oxidizing atmosphere so as to result in said zirconium being implanted as zirconia. 8. A composition according to claim 4, wherein said surface has been artificially oxidized to produce an oxide surface. 9. A composition according to claim 8, wherein said surface has been artificially oxidized by means selected from the group consisting of thermal means and chemical means. 10. A method of producing a cobalt chrome alloy, said method comprising implanting a substrate of cobalt chrome alloy with zirconium ions so as to produce said cobalt chrome alloy implanted with zirconium. 11. A method according to claim 10 wherein said zirconium ions for implanting said substrate of cobalt chrome alloy are obtained by ionizing a source of pure zirconium so as to produce zirconium ions, extracting and accelerating said zirconium ions towards a suitable cobalt chrome alloy substrate. 12. A method according to claim 11 , wherein said cobalt chrome alloy implanted with zirconium is further subjected to a step of coating with zirconium. 13. A method according to claim 11 , wherein said pure zirconium is implanted in an oxidizing atmosphere so as to result in said zirconium being implanted as zirconia. 14. A method according to claim 13, wherein said surface has been artificially oxidized to produce an oxide surface. 15. A method according to claim 14, wherein said surface has been artificially oxidized by means selected from the group consisting of thermal means and chemical means. 16. An article of manufacture comprising a cobalt chrome alloy implanted with zirconium ions. 17. An article according to claim 16, wherein said article is an orthopedic device. 18. An article according to claim 17 and including also a second part to be used together with said article of claim 17 in an opposing friction couple against said article of claim 17. 19. An article according to claim 18, wherein said second part comprises ultra-high molecular weight polyethylene. 20. An article according to claim 19, wherein said article is a knee implant. 21. An article according to claim 20, wherein said article is a femoral head.

Inventors: HIGHAM PAUL A, WARFIELD LARRY T, HIGHAM, PAUL, A., WARFIELD, LARRY, T.
Applicants: HOWMEDICA, HOWMEDICA INC.
Application year: 1994
Main classification: A61L27
IPC codes: A61F2, A61L27, C23C8, C23C14, C23C28